The opening and closing of heart valves occur primarily as a result of pressure differences. For example, the opening and closing of the mitral valve occurs as a result of the pressure differences between the left atrium and the left ventricle. During ventricular diastole, the venous return of blood from the pulmonary veins into the left atrium causes the pressure in the atrium to exceed that in the left ventricle. As a result, the mitral valve opens and allows blood to enter the left ventricle. As the left ventricle contracts during ventricular systole, the intraventricular pressure rises above the pressure in the atrium and pushes the mitral valve shut.
When the high pressure produced by contraction of the left ventricle pushes the valve leaflets too much, the leaflets become everted and prolapse results. This is normally prevented by contraction of the papillary muscles within the left ventricle, which are connected to the mitral valve leaflets by the chordae tendineae (chords). Contraction of the papillary muscles is simultaneous with the contraction of the left ventricle and serves to keep healthy mitral valve leaflets tightly shut at peak contraction pressures.
Mitral valve malfunction can stem from a variety of etiologies. For example, the causes of mitral regurgitation can range from intrinsic disease of the leaflets (e.g., mainly due to degenerative disease in patients with mitral valve prolapse), to functional mitral regurgitation (FMR), in which the valve is anatomically normal but stretched due to tethering and annular dilatation. Although mitral regurgitation in intrinsic disease occurs initially as leaflet disease, secondary annular dilatation occurs in the large majority of patients by the time they present for treatment. The larger proportion of patients with mitral regurgitation includes those without intrinsic disease of the leaflets, i.e., FMR.
Surgical correction of FMR is based upon overcorrection of concomitant annular dilatation using an undersized, complete, and rigid annuloplasty ring that is intended to reduce the diameter of the mitral annulus and allow for leaflet coaptation. Although complete correction of mitral regurgitation has been surgically demonstrated, an important recurrence of mitral regurgitation after annuloplasty valve repair is common (25%) because the left ventricle continues to dilate or remodel, thereby causing further tethering of the mitral leaflets.